This invention relates to a method by which ferromagnetic materials can be deposited by the magnetron sputtering technique.
There are a number of techniques used in industry for the deposition of thin films. The specific technique selected depends upon the particular application and might include high vacuum evaporation, chemical vapor deposition, plating, or sputtering. Each of these techniques offers some unique advantages. For example, almost any material can be deposited by sputtering, including both metals and insulators. With sputtering, alloys and mixtures can be deposited without fractionation and with the assurance that film composition will remain consistant from run to run. The source material will last for a large number of depositions without replenishment. Also, a further advantage of certain modes of sputtering is that the deposition is done at a relatively low temperature so that films can be applied to a number of materials that would be adversely affected by the temperatures that might be encountered with other deposition methods.
Two problems associated with classical sputter deposition are the low deposition rate and the poor power efficiency. For the most part, these problems have been overcome by the recent advances in high rate magnetic assisted or magnetron sputtering. With magnetron sputtering, magnets are located behind the cathode target in such a manner as to cause closed magnetic field loops to cut through the cathode. A portion of the magnetic field loop is in front of the front face of the cathode. The combination of magnetic field and electric field causes electrons to spiral in long confined paths giving rise to a very dense plasma immediately adjacent to the face of the target material. This dense plasma facilitates an increased yield of material sputtered from the target.
One limitation to magnetron sputtering, however, is that this technique is not amenable to the deposition of ferromagnetic materials. A target of ferromagnetic material would act as a shunt and would prevent magnetic field lines from cutting through the target and being located, as required, in front of the target. Therefore, materials such as iron or nickel cannot be magnetron sputtered. The only exception to this is that some limited success has been achieved by using specially fabricated targets in which a thin (say, about one-sixteenth of an inch) layer of the ferromagnetic material has been plated on a non-ferromagnetic base material. The layer is thin enough so as not to completely shunt the magnetic field. Use of such a target, however, defeats some of the advantages of sputtering. For example, the target is now very expensive and the source lifetime is severely limited because of the reduced amount of source material.
Some of the ferromagnetic materials have very desirable properties which render them valuable in many thin film applications. Nickel, for example, has found wide acceptance in the semiconductor industry. Accordingly, a need has existed for an improved method for the magnetron sputtering of ferromagnetic materials that would preserve the desired nonmagnetic properties of the materials.